Fix for non-empty a[name] tags in openJdk based javadocs.

openJdk based android javadoc contains <a name=... tags
that are non-empty. Currently they are being hidden,
causing badly formatted pages for some java.* classes
(missing text and random whitespace blocks).

This change makes the existing a[name] hiding rule
apply only to empty tags.

Test: make docs
Bug: 31700998
Change-Id: Ifa75bbabe308d7aed0cce6165c1113e5a382a121